In recent years, such automatic devices for processing paper currencies and bills or various marketable securities as currency count machines, currency detectors, and automatic teller machines etc. have been spread widely, and in these devices, there are scanning devices for identifying whether paper bills including paper currencies etc. are genuine or counterfeit.
These scanning devices identify whether paper bills are genuine or counterfeit through detecting reflected images or transmitted images, e.g. authenticity detection performed by means of scanning secondary light excited by an ultraviolet light source or an infrared light source etc. The secondary light refers to fluorescent light, phosphorescent light and infrared light etc. excited and generated by irradiation of irradiation light on printing ink and paper.
However, there are various anti-counterfeit technologies of paper bills in countries all over the world, and various kinds of printing ink on paper bills react differently to different lights. FIG. 1 is a sampling sequence diagram of an image scanning device in the prior technologies. As shown in FIG. 1, red light, green light and blue light are lit up in turn and sampled line by line during the sampling process. FIG. 2 is a sampling sequence diagram of a transmission scanning method in the prior technologies. The scanning principle of the transmission scanning method is that: light emitted by a transmission light source penetrates through an original copy (paper bill), which passes through a lens and is focused on a photosensitive sensor. FIG. 3 is a schematic diagram of brightness acquired according to the sampling sequence in FIG. 2. It is found by the inventor of the application that in the transmission scanning method, since there is transmission light source irradiating on the lens directly in an area without the original copy going through, output saturation will appear on the photosensitive sensor because of direct irradiation of the light source, thus generating an incidental image and reducing the scanning accuracy of paper bills. FIG. 4 is a schematic diagram of an incidental image generated by a photosensitive sensor due to output saturation. As shown in FIG. 4, a distortion zone S1 appears when an original copy is scanned and the ideal value of the gray scale of the distortion zone S1 should be 0. However, the actual gray scale of the distortion zone S1 is larger than 0 due to output saturation. FIG. 5 is a sampling sequence diagram of a reflection scanning method in the prior technologies. The scanning principle of the reflection scanning method is that: ultraviolet light emitted by a reflection light source irradiates on an original copy to excite secondary light which passes through a lens and is focused on a photosensitive sensor. FIG. 6 is a schematic diagram of brightness acquired according to the sampling sequence in FIG. 5. It is found by the inventor of the application that: in the reflection scanning method, phosphorescence may be generated on some paper bills irradiated by a ultraviolet light source and such phosphorescence will disappear after a very long period of time. The afterglow of the phosphorescence will disturb other colors, which will also reduce the scanning accuracy of paper bills. In addition, the problem of relatively low brightness of the excited secondary light also exists in the reflection scanning solution, and a secondary light image scanned according to the existing line-by-line sampling and white sample (Optical Density (OD) smaller than 0.1) compensation method has poor contrast and the scanned image is not clear, thus resulting in a failure in acquiring a paper bill anti-counterfeit image accurately to further affect identification of a genuine paper bill from a counterfeit one. FIG. 7 shows a schematic diagram of an image scanned according to the sampling sequence in FIG. 5. The ideal value of the gray scale of an anti-counterfeit image S2 in FIG. 7 should be 255. However, the actual gray scale of the anti-counterfeit image S2 is smaller than 255 due to poor contrast of the scanned secondary light image.
Currently, there is no effective solution for the problem of low scanning accuracy of paper bills in the related technologies.